tuple (x,y) is to be deleted so let's take it into relation.
Now we have to observe the effect on tuple (z,w) so we will take it also in our table.
So till now, we are having two tuples, (x,y) and (z,w). Now let's consider each option:
a) z>y means (y,z) tuple is there. So deletion of (x,y) doesn't give any cascade relation between (x,y),(z,w) and (y,z)
b) z>x means (x,z) exists in relation. But since 'ib' is primary key, we can't repeat 'x' .
c) w<x means (w,x) exists. On deleting (x,y), (w,x) should be deleted and then (z,w) to maintain DELETE CASCADE.
Hence option C is correct choice